PROJECT SUMMARY/ABSTRACT Age-related difficulties in spatial navigation pose a significant risk to older adults' independence, safety and confidence in travelling to new environments. Nonetheless, most of the existing literature documenting age effects on route learning has only examined traversing a route in one direction to a target location, which ignores that a route is often traveled both to and back from a destination. While limited research does indicate greater age-related deficits in route reversal than in repeating the route in the same direction, there is a gap in understanding the cognitive and neural determinants of route reversal performance in older adults. Thus, the proposed research will examine the influence of three targeted cognitive processes, as well as brain structures, on older adults' difficulty in route reversal. Learning a route in one direction involves development of a caudate- based egocentric representation of a series of body turns. However, route reversal may require a more flexible, hippocampal-dependent representation of the spatial relationships amongst features of the environment (i.e., an allocentric representation). In addition, route reversal may place greater demands upon the ability to mentally transform one's viewpoint (egocentric perspective taking). Knowledge of the reverse ordering of landmarks along the route (reverse temporal sequencing) could also be particularly relevant for route reversal. Both egocentric perspective taking and temporal sequencing are also in part associated with the hippocampus. Each of these abilities that may be particularly associated with route reversal evidence decline with advancing age, and the magnitude of hippocampal atrophy exceeds that of caudate atrophy. Thus, our underlying hypotheses are that a) greater contributions of an allocentric representation, egocentric perspective taking and reverse temporal sequencing to route reversal are determinants of greater age effects on reversal; and b) atrophy of the hippocampus contributes to differential age effects in route reversal. Across three experiments, younger and older adults will a) traverse a route to and from a target destination in virtual environments after learning the route using various methods, and b) complete independent measures of the targeted cognitive abilities. The proposed research will a) test the effectiveness of targeted interventions during learning in reducing age effects on route reversal; b) test the role of individual differences in the targeted cognitive abilities in age effects on route reversal; and c) test the role of MRI-based measures of hippocampus and caudate in age effects on route reversal. The proposed research will provide novel insights regarding the neural substrates of route reversal as well as factors that enhance route reversal ability in older adults. Identification of such factors will afford new opportunities for designing navigational aids targeting route presentation to improve this critical navigation skill in an aging population. Importantly, the investigation of route reversal in healthy aging can lead to insight into the earliest stages of Alzheimer's disease, which entails early atrophy of hippocampal circuits as well as allocentric navigation deficits in the preclinical phase.